Mendelian Genetics and Population Genetics: Study Guide

Mendelian and Human Genetics

Key Concepts in Mendelian Genetics

Mendelian genetics explores how traits are inherited from one generation to the next, based on the pioneering work of Gregor Mendel. Understanding these principles is fundamental to the study of heredity and variation in living organisms.

• Blending Theory of Inheritance: An outdated theory suggesting offspring are a blend of parental traits. Disproven by Mendel's work.

• Particulate Theory of Inheritance (Mendel’s Theory): Traits are inherited as discrete units (genes), not blended.

• Generational Terms:

  • Parental (P) Generation: The original pair of organisms crossed.

  • F1 Generation: First filial generation, offspring of the P generation.

  • F2 Generation: Second filial generation, offspring of the F1 cross.

• Allele: Alternative forms of a gene. Can be dominant (expressed if present) or recessive (expressed only if both alleles are recessive).

• Gene: Mendel defined a gene as a unit of inheritance controlling a trait.

• Diploid: Cells with two sets of chromosomes (2n).

• Haploid: Cells with one set of chromosomes (n), such as gametes.

• Genotype: The genetic makeup of an organism (e.g., AA, Aa, aa).

• Homozygous Dominant: Two dominant alleles (e.g., AA).

• Homozygous Recessive: Two recessive alleles (e.g., aa).

• Heterozygous: One dominant and one recessive allele (e.g., Aa).

• Phenotype: Observable traits of an organism.

• Segregation of Alleles: During gamete formation, alleles separate so each gamete receives one allele.

• Incomplete Dominance: Neither allele is fully dominant; heterozygotes show an intermediate phenotype.

• Codominance: Both alleles are fully expressed in the heterozygote (e.g., AB blood type).

• Multiple Alleles: More than two possible alleles exist for a gene (e.g., ABO blood group).

• Laws of Probability in Genetic Crosses: Used to predict the likelihood of offspring genotypes and phenotypes.

• Single Factor Cross: Cross involving one gene.

• Double Factor Cross (Dihybrid Cross): Cross involving two genes; demonstrates the Law of Independent Assortment (genes on different chromosomes assort independently during meiosis).

Human Genetics

• Karyotype: A visual representation of all chromosomes in a cell, used to detect chromosomal abnormalities.

• Autosomes: The 22 pairs of chromosomes not involved in sex determination.

• Sex Chromosomes: X and Y chromosomes determining biological sex.

• Nondisjunction: Failure of chromosomes to separate properly during meiosis, leading to aneuploidy (e.g., Down syndrome).

• Autosomal Dominant Inheritance: Trait appears in every generation; only one copy of the allele needed for expression.

• Autosomal Recessive Inheritance: Trait appears only when two recessive alleles are present.

• X-linked Recessive Inheritance: Trait more common in males; females must have two copies to express the trait.

• X-linked Dominant Inheritance: Trait expressed if at least one dominant allele is present on the X chromosome.

• Mitochondrial Inheritance: Traits inherited through mitochondrial DNA, passed from mother to offspring.

• Pedigrees: Diagrams showing inheritance patterns across generations; used to infer genotypes and modes of inheritance.

Major Concepts and Problem-Solving

• Chromosome Structure: Each chromosome consists of two chromatids (identical DNA molecules) joined at a centromere, containing thousands of genes.

• Genetic Crosses: Ability to perform and interpret single and double factor crosses involving incomplete dominance, codominance, multiple alleles, and X-linked traits.

• Pedigree Analysis: Interpreting inheritance patterns and predicting genotypes from family trees.

Population Genetics and Microevolution

Evolution by Natural Selection

Population genetics studies the genetic composition of populations and how it changes over time, forming the basis for understanding microevolution and natural selection.

• Darwin and Wallace’s Theory: Evolution occurs by natural selection, where environmental pressures and natural variation lead to differential survival and reproduction.

• Microevolution: Short-term changes in allele frequencies within a population.

• Macroevolution: Long-term, large-scale evolutionary changes, such as speciation.

Sources of Genetic Variation

• Mutations: Changes in DNA sequence; can be harmful, neutral, or beneficial.

• Sexual Reproduction: Shuffles existing alleles through independent assortment, fertilization, and crossing over.

• Nondisjunction and Chromosomal Abnormalities: Errors during meiosis can introduce variation.

Gene Pool and Allele Frequency

• Gene Pool: All alleles present in a population.

• Allele Frequency: Proportion of a specific allele among all alleles for a gene in a population.

• Measurement: Can be estimated from observable traits, protein/DNA sequences.

Hardy-Weinberg Equilibrium

The Hardy-Weinberg Law provides a mathematical model for genetic equilibrium in a population, assuming no evolutionary forces are acting.

• Allele Frequency Equation:

  • $p + q = 1$

• Genotype Frequency Equation:

  • $p^2 + 2pq + q^2 = 1$

• Steps for Calculating Frequencies:

  1. Determine frequency of homozygous recessive genotype ($q^2$) from phenotype data.

  2. Calculate $q$ by taking the square root of $q^2$.

  3. Find $p$ using $p = 1 - q$.

  4. Calculate genotype frequencies: $p^2$ (homozygous dominant), $2pq$ (heterozygous), $q^2$ (homozygous recessive).

  5. Check that $p^2 + 2pq + q^2 = 1$.

• Requirements for Equilibrium:

  • Large population size

  • Random mating

  • No mutation

  • No natural selection

  • No migration (emigration/immigration)

Factors Disrupting Genetic Equilibrium

• Mutation: Introduces new alleles.

• Migration (Gene Flow): Movement of alleles between populations.

• Genetic Drift: Random changes in allele frequencies, especially in small populations.

  • Bottleneck Effect: Sharp reduction in population size due to a catastrophic event.

  • Founder Effect: New population established by a small group, leading to different allele frequencies.

• Natural Selection: Differential survival and reproduction changes allele frequencies.

Types of Natural Selection

• Stabilizing Selection: Favors average phenotypes.

• Directional Selection: Favors one extreme phenotype (e.g., dark fur in rock pocket mice).

• Disruptive Selection: Favors both extremes over the average.

• Sexual Selection: Favors traits that increase mating success.

Application: Hardy-Weinberg and Natural Selection

• The Hardy-Weinberg equation can be used to analyze real populations, such as the rock pocket mice, to study the effects of selection on allele frequencies.

• Selection coefficients can be incorporated into the model to account for fitness differences among genotypes.

Practice Data: Allele and Genotype Frequencies

The following table summarizes class data for several traits, showing genotype counts, allele frequencies, and genotype frequencies. This data can be used to practice Hardy-Weinberg calculations.

Trait	Genotype	Class Data	Allele Frequency	Genotype Frequency
Free earlobe	EE, Ee	26	p=0.54	p2=0.29
Attached earlobe	ee	7	q=0.46	2pq=0.50 q2=0.21
Non hitchhiker’s thumb	HH, Hh	13	p=0.22	p2=0.05
Hitchhiker’s thumb	hh	20	q=0.78	2pq=0.34 q2=0.61
Mid-digital hair	MM, Mm	10	p=0.17	p2=0.03
No mid-digital hair	mm	23	q=0.83	2pq=0.28 q2=0.70
Tongue rolling	RR, Rr	31	p=0.75	p2=0.56
No tongue rolling	rr	2	q=0.25	2pq=0.38 q2=0.06
Right handed	AA, Aa	30	p=0.70	p2=0.49
Left handed	aa	3	q=0.30	2pq=0.42 q2=0.09
Widow’s peak	WW, Ww	16	p=0.28	p2=0.08
No widow’s peak	ww	17	q=0.72	2pq=0.40 q2=0.52
Dimples	DD, Dd	4	p=0.06	p2=0.004
No dimples	dd	29	q=0.94	2pq=0.113 q2=0.88

Additional Practice Problems

Practice calculating allele and genotype frequencies using the Hardy-Weinberg equations with the following data:

Trait	Genotype	Class Data	Allele Frequency	Genotype Frequency
Darwin tubercle	DD, Dd	17	p=0.30	p2=0.09
No Darwin tubercle	dd	16	q=0.70	2pq=0.42 q2=0.48
PTC Tasting	TT, Tt	23	p=0.45	p2=0.20
PTC Non-tasting	tt	10	q=0.55	2pq=0.50 q2=0.30
Cleft chin	CC, Cc	1	p=0.02	p2=0.0004
Smooth chin	cc	32	q=0.98	2pq=0.039 q2=0.97

Example Calculation

• If 10 out of 100 students have attached earlobes (homozygous recessive, ee):

  • Frequency of ee ($q^2$) = 10/100 = 0.10

  • $q = \sqrt{0.10} = 0.32$

  • $p = 1 - 0.32 = 0.68$

  • Genotype frequencies:

    • $p^2 = (0.68)^2 = 0.46$ (homozygous dominant)

    • $2pq = 2 \times 0.68 \times 0.32 = 0.44$ (heterozygous)

    • $q^2 = 0.10$ (homozygous recessive)

Summary

• Understand Mendelian principles and inheritance patterns.

• Be able to perform and interpret genetic crosses and pedigree charts.

• Apply Hardy-Weinberg equations to calculate allele and genotype frequencies.

• Recognize factors that disrupt genetic equilibrium and drive evolution.

Additional info: For more detailed problem-solving, refer to class notes and worksheets on genetic crosses and population genetics problems.